The present invention relates to an improvement in coating compositions, e.g. paints, lacquers and varnishes, or, more particularly, to an improvement in coating compositions capable of imparting the coated surface with decreased blocking as well as to a coating composition thus improved.
A problem generally encountered in a surface coated with a coating composition is that the surface is more or less susceptible to mechanical damages and stain or appearance of dullness in the luster when the surface is contacted with other solid bodies. Such undesirable phenomena are more remarkable when the coated surface is formed of a coating composition giving a coating film of low mechanical strengths.
For example, plywood boards coated with a lacquer or an aminoalkyd or urethane resin-based paint and colored iron sheets coated with an acrylic paint have a problem of so-called blocking to adhere to each other when they are stacked in layers one on the other and kept for a length of time. When such boards or sheets stored in layers are separated, the coating layers are more or less mechanically damaged or the luster of the surface is decreased. When a pressure-sensitive adhesive tape is applied to the coated surface and then peeled off, it is not rare that a part of the coating film is transferred to the adhesive tape resulting in a damaged surface. Further, a problem frequently encountered in the fabrication works of coated plywood boards using an adhesive is that the coating film is sometimes carried away by the adhesive when the surface is smeared with the adhesive and the adhesive is removed by wiping.
Various proposals and attempts have been made hitherto to solve the above described problem of defects in the coating films of conventional coating compositions. For example, Japanese Patent Publication 47-18985 teaches admixture of a coating composition with a dimethyl silicone fluid or a polyether-modified organopolysiloxane copolymer, Japanese Patent Publication 48-4367 teaches addition of an aminoalkyl-containing organopolysiloxane copolymer and Japanese Patent Publication 53-20530 teaches addition of a mercaptoalkyl-containing organopolysiloxane copolymer.
Each of these methods, however, has a problem in its part. For example, the method of the addition of a dimethylsilicone fluid is little effective in preventing the phenomenon of blocking in addition to the disadvantage of increased cissing or crawling although the method is considerably effective in the improvement of levelling of the coating layer on the surface, prevention of flooding and the like. The method of the addition of a polyether-modified organopolysiloxane copolymer or an aminoalkyl-containing organopolysiloxane copolymer is free from the disadvantage of increased cissing or crawling but not so effective as desired in preventing the phenomenon of blocking and the coating film formed of a coating composition containing such an additive is liable to be peeled when the coated surface is prolongedly kept with an adhering pressure-sensitive adhesive tape bonded thereto or when a pressure-sensitive adhesive tape is repeatedly applied to and peeled from the same area of the surface. The method of the addition of a mercaptoalkyl-containing organopolysiloxane copolymer is little effective for a urethane-based coating composition although the method is effective for the coating compositions of other types.